In conventional plumbing installations, where no heat recovery unit is present, used hot gray water originating from a shower or a bath flows to the drainage system and the thermal energy still present in the gray water is lost. Although gray water heat recovery models are available on the market, they are usually installed vertically and are only located on the main drain of the building. This location greatly limits heat recovery from gray water due to the fact that the heat accumulated in the apparatus is quickly lost by the sudden arrival of several liters of very cold water coming from toilets, sinks and other hardware.
Canadian Patent Application CA 2,903,527 (Gil et al.) discloses a counter-current gray water heat recovery apparatus for pre-heating fresh water by passing it through coil or linear tubing modules immersed in a gray water sump. The heat exchanger has a first side defining a gray water flow path, and a second side defining a fresh-water flow path. The gray water path is a gravity-feed flow path, while the fresh water flow path is a pressure-feed flow path. The use of coils or linear tubing modules can be inconvenient and result in significant maintenance issues, as the coils or linear tubing modules need to be cleaned or replaced. In addition, the use of this system in domestic settings may be limited by the costs involved in setting it up and operating it.
French Patent Application FR 2,970,071 (Duong) discloses an apparatus for recovering heat from gray water comprising a cylindrical conical container filled with gray water surrounded by fresh water. In this arrangement, since the fresh water pressure is higher than the gray water pressure inside the container, the cylindrical portion must be strong enough to withstand the external pressure from the fresh water. This can significantly impact the operating costs of this type of system.
German Patent Application DE 10201501477 (Molle et al.) discloses a gray water heat recovery apparatus whereby a gray water tank is surrounded by a fresh water conduit before it is heated prior to consumption. In this case, thermal energy recovery is limited by the relatively large size of the gray water container.
There is thus a need for a simple and efficient gray water heat recovery system that can help reduce energy costs in domestic as well as public buildings.